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System.Data.SqlClient.SqlException : The ntext data type cannot be selected as DISTINCT because it is not comparable

After upgrading to NHibernate 5, one of our integration tests started to fail with the following error message:

System.Data.SqlClient.SqlException : The ntext data type cannot be selected as DISTINCT because it is not comparable.

The error itself happened inside a by Nhibernate generated query:

Message: NHibernate.Exceptions.GenericADOException : [SQL: select distinct supplier1_.SupplierID as supplierid1_138_, supplier1_.Address as address2_138_, supplier1_.City as city3_138_, supplier1_.CompanyName as companyname4_138_, supplier1_.ContactName as contactname5_138_, supplier1_.ContactTitle as contacttitle6_138_, supplier1_.Country as country7_138_, supplier1_.Fax as fax8_138_, supplier1_.HomePage as homepage9_138_, supplier1_.Phone as phone10_138_, supplier1_.PostalCode as postalcode11_138_, supplier1_.Region as region12_138_ from Products product0_ inner join Categories category2_ on product0_.CategoryID=category2_.CategoryID, Suppliers supplier1_ where supplier1_.SupplierID=product0_.SupplierID and category2_.CategoryName=@p0] 

The error message pointed us to a ntext data type. A look into the database revealed that indeed we were using the ntext data type.

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It is recommended to avoid this data type(it is marked as obsolete) and use nvarchar(MAX) instead.

But why didn’t we got this error before? It turned out that the query generated by NHibernate 5 is different then the one used by NHibernate 4. In NHibernate 5 an extra DISTINCT clause is added to the query, resulting in the error message above.

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